Some designs of wireless communication devices—such as smart phones, tablet computers, and laptop computers—contain one or more Subscriber Identity Module (SIM) cards that provide users with access to multiple separate mobile telephony networks. Examples of mobile telephony networks include Third Generation (3G), Fourth Generation (4G), Long Term Evolution (LTE), Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Code Division Multiple Access (CDMA), Wideband CDMA (WCDMA), Time Division Synchronous CDMA (TD-SCDMA), Global System for Mobile Communications (GSM), Universal Mobile Telecommunications Systems (UMTS), evolved High Speed Packet Access (HSPA+), Dual-Cell High Speed Packet Access (DC-HSPA), Evolution Data-Optimized (EV-DO), Enhanced Data rates for GSM Evolution (EDGE), and single carrier Radio Transmission Technologies (1×RTT).
To enable communications in various networks, a wireless communication device may process data to be transmitted over a radio frequency (RF) resource and may perform complementary processing on received RF signals to recover data. In particular, data transmission may involve encoding and modulating the data, generating data samples, and conditioning the samples to generate an output RF signal. Wireless communication devices typically include a power amplifier (PA) to provide high transmit power for the output RF signal. The PA amplifies the RF signal to a desired level for transmission, which may depend on how far the user is away from a base station.
Since power amplification consumes power, techniques to improve the efficiency of power amplifiers may be implemented in wireless communication devices in order to prolong operation on a battery charge. Such techniques may include adjusting the power supplied to the power amplifier so that the applied power tracks the amount of power in the transmit signal. Adjusting the applied power based on the transmit signal is referred to generally as envelope tracking (ET) mode and there are different forms or modes of envelope tracking that can be implemented. Another mode of operation is average power tracking (APT) mode, in which the applied power is adjusted on a per transmission slot basis. Thus in ET mode, the applied power constantly fluctuates as the signal amplitude fluctuates while in APT mode the applied power is constant within each slot, and changes discretely between slots.
A wireless communication device that includes one or more SIMs and connects to two or more separate mobile telephony networks using one or more shared RF resources/radios may be termed a multi-SIM-multi-standby (MSMS) wireless communication device. One example is a dual-SIM-dual-standby (DSDS) communication device, which includes two SIM cards/subscriptions that are each associated with a separate radio access technology (RAT), and the separate RATs share one RF resource chain to communicate with two separate mobile telephony networks on behalf of their respective subscriptions. When one subscription is using the RF resource, the other subscription is in stand-by mode and is not able to communicate using the RF resource.
One consequence of having a plurality of subscriptions that maintain network connections is that the subscriptions may sometimes interfere with each other's communications. For example, two subscriptions on a DSDS communication device utilize a shared RF resource to communicate with their respective mobile telephony networks, and only one subscription may use the RF resource to communicate with the subscription's mobile network at a time. Even when a subscription is in an idle or standby mode, meaning that the subscription is not actively communicating with the network, the subscription may still need to periodically receive access to the shared subscription resource in order to perform various network operations. For example, an idle subscription may need the shared RF resource at regular intervals to perform idle-mode operations to receive network paging messages in order to remain connected to the network, etc.
In conventional wireless communication devices, the subscription actively using an RF resource that is shared with an idle RAT may occasionally be forced to interrupt the active subscription's RF operations so that the idle subscription may use the shared RF resource to perform the idle subscription's idle-standby mode operations (e.g., paging monitoring and decoding, cell reselection, system information monitoring, etc.). This process of switching access of the shared RF resource from the active subscription to the idle subscription is sometimes referred to as a “tune-away,” as the RF resource tunes away from the active subscription's frequency band or channel and tune to the idle subscription's frequency bands or channels. After the idle subscription has finished network communications, access to the RF resource may switch from the idle subscription to the active subscription via a “tune-back” operation.
In some RF resources, the active subscription may still retain use of one or more transmitters in the RF resource during the tune-away to the idle subscription. Because the idle subscription only receives information from the network during a tune-away, the idle subscription may only need to utilize receivers in the RF resource and not transmitters. Thus, the transmitters may still be available to the active subscription to transmit information during the tune-away. However, the simultaneous operation of the transmitter and the receiver in the RF resource may result in interference effects that degrade the reception ability of the receiver. Generally, receiver desensitization (sometimes referred to as “de-sense”), or degradation of receiver sensitivity, may result from noise interference of a nearby transmitter caused by transmitter harmonics, reception band noise, or other sources. This interference may be especially pronounced when the transmitter is operating in ET mode because the continuous power fluctuations amplifies noise effects and generates more interfering harmonics. Thus then a tune-away occurs, operation of the transmitter in ET mode by the active subscription may cause significant de-sense with the receiver used by the idle subscription to receive paging information.